Russia's 2014 annexation of Crimea radically changed the calculus between the West and Russia that has defined the last 25 years. NATO is now trying to reassure a nervous Eastern Europe and deter Moscow from new aggression. This level of tension hasn't been felt in a generation. In the final installment of their weeklong series on Eastern Europe for PBS NewsHour, Pulitzer Center grantees Nick Schifrin and Zach Fannin report on U.S. and NATO troop buildup from Szczecin, Poland.